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|United States Patent Application
Bianchi, John R.
;   et al.
June 27, 2002
Open intervertebral spacer
Open chambered spacers, implanting tools and methods are provided. The
spacers 500' include a body 505' having a wall 506' which defines a
chamber 530' and an opening 531' in communication with the chamber 530'.
In one embodiment the wall 506' includes a pair of arms 520', 521' facing
one another and forming a mouth 525' to the chamber 530'. Preferably, one
of the arms 520' is truncated relative to the other, forming a channel
526. In one aspect the body 505' is a bone dowel comprising an off-center
plug from the diaphysis of a long bone. The tools 800 include spacer
engaging means for engaging a spacer and occlusion means for blocking an
opening defined in the spacer. In some embodiments, the occlusion means
820 includes a plate 821 extendable from the housing 805. In one specific
embodiment the plate 821 defines a groove 822 which is disposed around a
fastener 830 attached to the housing 805 so that the plate 821 is
slideable relative to the housing 805.
Bianchi, John R.; (Gainesville, FL)
; Carter, Kevin C.; (Gainesville, FL)
; Estes, Bradley T.; (Memphis, TN)
; Boyd, Larry; (Memphis, TN)
; Pafford, John A.; (Germantown, TN)
Woodard, Emhardt, Naughton,
Moriarty and McNett
Bank One Center/Tower
111 Monument Circle, Suite 3700
December 28, 2001|
|Current U.S. Class:
||623/17.16; 623/17.11; 623/911; 623/925 |
|Class at Publication:
||623/17.16; 623/17.11; 623/911; 623/925 |
What is claimed is:
1. A hollow intervertebral spacer, comprising: an elongated body having an
outer surface and a longitudinal axis along a length of said body and
defining a chamber therethrough along a second axis substantially
perpendicular to said longitudinal axis; a first arm connected to said
body; an opposite second arm connected to said body and facing said first
arm; and said first arm and said second arm forming a mouth to said
2. The spacer of claim 1 wherein said body further comprises: a tool
engaging end defining a tool engaging hole for receiving a driving tool
for implanting the spacer.
3. The spacer of claim 2 wherein said anterior surface further defines a
slot surrounding said tool engaging hole.
4. The spacer of claim 1 wherein said outer surface defines threaded bone
5. The spacer of claim 1 wherein said wall is curved and said chamber is
6. The spacer of claim 1 wherein said body is composed of a porous
7. The spacer of claim 1 wherein said body is composed substantially of
8. The spacer of claim 1 wherein said first arm is truncated relative to
said second arm.
9. The spacer of claim 3 wherein said outer surface defines threaded bone
engaging portions and said body is composed of cortical bone.
10. The spacer of claim 4 wherein said spacer is a bone dowel obtained
from the diaphysis of a long bone having a medullary canal, said chamber
including a portion of the canal.
11. The spacer of claim 1, further comprising an osteogenic material
packed within said chamber.
12. The spacer of claim 11 wherein said osteogenic material comprises
autograft, allograft, xenograft, demineralized bone, a calcium phosphate
material, a bioceramic, bioglass, an osteoinductive factor or mixtures of
13. An interbody fusion spacer, comprising: a body having a wall defining
a chamber, the body defining an opening in communication with said
chamber, said wall having a first arm and an opposite second arm facing
said first arm, said first arm and said second arm forming a mouth to
said chamber, wherein said first arm is truncated relative to said second
14. The spacer of claim 13 wherein said body further comprises: a tool
engaging end defining a tool engaging hole for receiving a driving tool
for implanting the spacer.
15. The spacer of claim 14 wherein said anterior surface further defines a
slot surrounding said tool engaging hole.
16. The spacer of claim 13 wherein said body further comprises: an outer
surface defining threaded bone engaging surfaces.
17. The spacer of claim 13 wherein said wall is curved and said chamber is
18. The spacer of claim 13 wherein said spacer comprises cortical bone.
19. The spacer of claim 13 further comprising an osteogenic material
packed within said chamber.
20. A graft comprising an elongated body consisting essentially of
cortical bone, said body having an outer surface and a longitudinal axis
along a length of said body and defining a chamber therethrough along a
second axis substantially perpendicular to said longitudinal axis, said
body further defining a channel defined along said longitudinal axis and
in communication with said chamber and said outer surface.
21. The graft of claim 20 wherein said outer surface defines threaded bone
22. The graft of claim 20 further comprising an osteogenic material packed
within said chamber.
23. A hollow intervertebral spacer, comprising: a cylindrical body having
a wall, said wall having an outer surface and defining a chamber and an
opening in communication with said chamber; and a channel defined in said
wall in communication with said chamber and said outer surface.
24. The spacer of claim 23 wherein said outer surface defines threaded
bone engaging portions.
25. A bone graft having a C-shaped wall defining a chamber.
26. The spacer of claim 25 wherein said graft is a bone dowel obtained
from the diaphysis of a long bone having a medullary canal, said chamber
including a portion of the canal.
27. A "C"-shaped dowel substantially composed of cortical bone.
28. The "C"-shaped dowel of claim 27 comprising a bone plug obtained from
the diaphysis of a long bone, said dowel having a substantially
29. The "C"-shaped dowel of claim 28 having a chamfered insertion end.
30. The "C"-shaped dowel of claim 28 further comprising a tool engaging
end defining an instrument attachment hole.
31. The "C"-shaped dowel of claim 30 wherein the tool engaging end also
defines a driver slot surrounding said hole.
32. The "C"-shaped dowel of claim 28 further comprising am external
feature machined into an outer surface of the dowel.
33. The "C"-shaped dowel of claim 32 wherein said feature includes a
34. The "C"-shaped dowel of claim 32 wherein said feature includes threads
formed along a portion of the length of the dowel.
35. The "C"-shaped dowel of claim 27 having a length of between about 8 mm
to about 36 mm.
36. The "C"-shaped dowel of claim 35 having a diameter of between about 10
mm and about 24 mm.
37. The "C"-shaped dowel of claim 28 further comprising an osteogenic
composition packed within said chamber.
38. The "C"-shaped dowel of claim 38 wherein said osteogenic composition
comprises autogenous bone, bone morphogenetic protein, a calcium
phosphate composition or a mixture of these.
39. The "C"-shaped dowel of claim 27 obtained as an off-center transverse
plug from the shaft of a donor's fibula, radius, ulna, humerus, femur or
40. A method of making a dowel which comprises machining an off-center
transverse plug from the diaphysis of a donor's fibula, radius, ulna,
humerus, femur or tibia, said plug having a diameter of between about 10
mm and about 24 mm and a depth (length) of between about 8 mm and about
30mm such that the resulting dowel has, running through it, perpendicular
to the long axis of the dowel, a substantially "C"-shaped chamber.
41. The method of claim 40 further comprising chamfering one end of said
plug to form a generally curved surface for ease of insertion of the
dowel into an intervertebral cavity.
42. The method of claim 40 further comprising machining an instrument
attachment hole into one end of the dowel.
43. The "C"-shaped dowel of claim 27 prepared by a process comprising
machining an off-center transverse plug from the diaphysis of a donor's
fibula, radius, ulna, humerus, femur or tibia, said plug having a
diameter of between about 10 mm and about 24 mm and a length of between 8
mm and about 36 mm such that the resulting dowel has, running through it,
perpendicular to the long axis of the dowel, a substantially "C"-shaped
44. The "C"-shaped dowel of claim 27 having an outer surface defining a
45. The "C"-shaped dowel of claim 44 wherein said feature includes a
46. The "C"-shaped dowel of claim 44 wherein said feature includes threads
formed along a portion of the length of the dowel.
47. The "C"-shaped dowel of claim 46 wherein said thread has a pitch of
48. A spacer insertion tool, comprising: a housing having a proximal end
and an opposite distal end and defining a passageway between said
proximal end and said distal end; a shaft having a first end and an
opposite second end, said shaft disposed within said passageway with said
first end adjacent said distal end, said first end defining a spacer
engager; and an occlusion member extendable from said distal end of said
housing for blocking an opening defined in the spacer when said spacer
engager is engaged to the spacer.
49. The tool of claim 48, further comprising a fastener attached to said
shaft and wherein said occlusion member includes a plate defining a
groove, said groove disposed around said fastener so that said plate is
slidable relative to said housing.
50. The tool of claim 49 wherein said plate has a curved superior surface
which approximates the outer surface of the spacer when said spacer
engaging means is engaged to the spacer and said occlusion means is
blocking the opening of the spacer.
51. The tool of claim 48 wherein said shaft is slidingly disposed within
52. The tool of claim 48 wherein said spacer engager is threaded for
mating engagement with a threaded hole in a spacer.
53. The tool of claim 48 wherein said spacer engager is a hex for mating
engagement with an internal hex in a spacer.
54. An insertion tool for inserting a spacer into an intervertebral space,
comprising: spacer engaging means for engaging the spacer; and occlusion
means separate from said spacer engaging means for blocking an opening
defined in the spacer.
55. The tool of claim 54 wherein said occlusion means includes a plate,
said plate having a curved superior surface which approximates the outer
surface of the spacer when said spacer engaging means is engaged to the
spacer and said occlusion means is blocking the opening of the spacer.
56. The tool of claim 54 wherein said spacer engaging means includes a
post for engaging a hole in the spacer.
57. The tool of claim 56 wherein said post is threaded for mating
engagement with a threaded hole in a spacer.
58. The tool of claim 56 wherein said post is a hex for mating engagement
with an internal hex in a spacer.
59. The tool of claim 74 wherein said spacer engaging means is a pair of
prongs having opposite, facing spacer engaging members for grasping an
outer surface of the spacer.
60. A driving tool for implanting an interbody spacer in a space between
adjacent vertebrae, the spacer including a body defining a chamber and an
opening in communication with the chamber, the body having a pair of arms
facing one another and forming a mouth to the chamber, and an anterior
surface defining a tool engaging hole, the tool comprising: spacer
engaging means for engaging the tool engaging hole; and occlusion means
for blocking said mouth.
61. The tool of claim 60 further comprising a housing and wherein said
occlusion means is extendable from said housing.
62. The tool of claim 60 wherein said spacer engaging means is a threaded
post for threading engagement with the tool engaging hole.
63. A method for fusing two adjacent vertebrae, comprising the steps of:
providing a spacer, the spacer including a body having a wall, said wall
having an outer surface and defining a chamber and an opening in
communication with said chamber, and a channel defined in said wall in
communication with said chamber and said outer surface; preparing the
vertebrae and the intervertebral space between the vertebrae to receive
the spacer; placing the spacer into the intervertebral space after the
preparing step so that the opening is in communication with at least one
of the vertebrae; and packing osteogenic material into the channel after
the placing step.
64. A method for fusing two adjacent vertebrae, comprising the steps of:
providing a spacer, the spacer including a body having a wall, said wall
having an outer surface and defining a chamber and an opening in
communication with said chamber, and a channel defined in said wall in
communication with said chamber and said outer surface; preparing the
vertebrae and the intervertebral space between the vertebrae to receive
the spacer; packing osteogenic material into the chamber; blocking the
channel; and placing the spacer into the intervertebral space after the
blocking step so that the opening is in communication with at least one
of the vertebrae.
65. The method of claim 64 further comprising: implanting a second spacer
into the intervertebral space after the placing step, the second spacer
having a body having a wall, said wall having an outer surface and
defining a chamber and an opening in communication with said chamber, and
a channel defined in said wall in communication with said chamber and
said outer surface; and orienting the first spacer and the second spacer
so that the channels of the first and second spacers face one another.
66. The method of claim 65 further comprising packing an osteogenic
material into the channels of the first and second spacers.
67. The method of claim 64, further comprising providing a tool of claim
27; engaging the spacer engager of the tool to the spacer; and wherein
the blocking step includes extending the occlusion member to block the
68. The spacer of claim 1 wherein said body is composed of a metal, a
ceramic, a polymer or a composite or alloy thereof.
69. The spacer of claim 13 wherein said body is composed of a metal, a
ceramic, a polymer or a composite or alloy thereof.
70. The spacer of claim 1 wherein said outer surface includes a curved
portion and a flattened portion.
71. The spacer of claim 13 wherein said body further comprises an outer
surface that defines a curved portion and a flattened portion.
FIELD OF THE INVENTION
 The present invention broadly concerns arthrodesis for stabilizing
the spine. More specifically, the invention provides open-chambered
intervertebral spacers, instruments for implanting the spacers and
methods for making and using the spacers.
BACKGROUND OF THE INVENTION
 Intervertebral discs, located between the endplates of adjacent
vertebrae, stabilize the spine, distribute forces between vertebrae and
cushion vertebral bodies. A normal intervertebral disc includes a
semi-gelatinous component, the nucleus pulposus, which is surrounded and
confined by an outer, fibrous ring called the annulus fibrosus. In a
healthy, undamaged spine, the annulus fibrosus prevents the nucleus
pulposus from protruding outside the disc space.
 Spinal discs may be displaced or damaged due to trauma, disease or
aging. Disruption of the annulus fibrosus allows the nucleus pulposus to
protrude into the vertebral canal, a condition commonly referred to as a
herniated or ruptured disc. The extruded nucleus pulposus may press on a
spinal nerve, which may result in nerve damage, pain, numbness, muscle
weakness and paralysis. Intervertebral discs may also deteriorate due to
the normal aging process or disease. As a disc dehydrates and hardens,
the disc space height will be reduced leading to instability of the
spine, decreased mobility and pain.
 Sometimes the only relief from the symptoms of these conditions is
a discectomy, or surgical removal of a portion or all of an
intervertebral disc followed by fusion of the adjacent vertebrae. The
removal of the damaged or unhealthy disc will allow the disc space to
collapse. Collapse of the disc space can cause instability of the spine,
abnormal joint mechanics, premature development of arthritis or nerve
damage, in addition to severe pain. Pain relief via discectomy and
arthrodesis requires preservation of the disc space and eventual fusion
of the affected motion segments.
 Bone grafts are often used to fill the intervertebral space to
prevent disc space collapse and promote fusion of the adjacent vertebrae
across the disc space. In early techniques, bone material was simply
disposed between the adjacent vertebrae, typically at the posterior
aspect of the vertebra, and the spinal column was stabilized by way of a
plate or rod spanning the affected vertebrae. Once fusion occurred, the
hardware used to maintain the stability of the segment became superfluous
and was a permanent foreign body. Moreover, the surgical procedures
necessary to implant a rod or plate to stabilize the level during fusion
were frequently lengthy and involved.
 It was therefore determined that a more optimal solution to the
stabilization of an excised disc space is to fuse the vertebrae between
their respective end plates, preferably without the need for anterior or
posterior plating. There have been an extensive number of attempts to
develop an acceptable intradiscal implant that could be used to replace a
damaged disc and maintain the stability of the disc interspace between
the adjacent vertebrae, at least until complete arthrodesis is achieved.
The implant must provide temporary support and allow bone ingrowth.
Success of the discectomy and fusion procedure requires the development
of a contiguous growth of bone to create a solid mass because the implant
may not withstand the compressive loads on the spine for the life of the
 Several metal spacers have been developed to fill the void formed
and to promote fusion. Sofamor Danek Group, Inc., (1800 Pyramid Place,
Memphis, Tenn. 38132, (800) 933-2635) markets a number of hollow spinal
cages. For example, U.S. Pat. No. 5,015,247 to Michelson and U.S. Ser.
No. 08/411,017 to Zdeblick disclose a threaded spinal cage. The cages are
hollow and can be filled with osteogenic material, such as autograft or
allograft, prior to insertion into the intervertebral space. Apertures
defined in the cage communicate with the hollow interior to provide a
path for tissue growth between the vertebral endplates.
 Although the metal fusion devices of Sofamor Danek and others are
widely and successfully employed for reliable fusions, it is sometimes
desirable to use an all-bone product. Bone provides many advantages for
use in fusions. It can be incorporated after fusion occurs and therefore
will not be a permanent implant. Bone allows excellent postoperative
imaging because it does not cause scattering like metallic implants.
Stress shielding is avoided because bone grafts have a similar modulus of
elasticity as the surrounding bone. Although an all-bone spacer provides
these and other benefits, the use of bone presents several challenges.
Any spacer which will be placed within the intervertebral disc space must
withstand the cyclic loads of the spine. Cortical bone products may have
sufficient compressive strength for such use, however, cortical bone will
not promote rapid fusion. Cancellous bone is more conducive to fusion but
is not biomechanically sound as an intervertebral spacer.
 Several bone dowel products such as the Cloward Dowel have been
developed over the years. Bone dowels in the shape of a generally
circular pin can be obtained by drilling an allogeneic or autogeneic plug
from bone. As shown in FIGS. 1 and 2, the dowels 100, 200 have one or two
cortical surfaces 110 and an open, latticed body of brittle cancellous
bone 120, 220 backing the cortical surface 210 or between the two
cortical surfaces 110. The dowels 100, 200 also include a drilled and/or
tapped instrument attachment hole 115, 215. Dowels and other bone
products are available from the University of Florida Tissue Bank, Inc.,
(1 Innovation Drive, Alachua, Fla. 32615, 904-462-3097 or 1-800-OAGRAFT;
Product numbers 280012, 280014, and 280015).
 While the bone dowels of the prior art are valuable bone grafting
materials, these dowels have relatively poor biomechanical properties, in
particular a low compressive strength. Accordingly, these dowels may not
be suitable as an intervertebral spacer without internal fixation due to
the risk of collapsing prior to fusion under the intense cyclic loads of
the spine. A need remains for dowels having the advantages of allograft
but with even greater biomechanical strength.
 In response to this need, the University of Florida Tissue Bank,
Inc., has developed a proprietary bone dowel machined from the diaphysis
of long bones. Referring now to FIG. 3, the dowel 300 includes a tool
engagement end 301 and an opposite insertion end 302. Between the two
ends 301 and 302, the dowel 300 includes a chamber 330 formed from the
naturally occurring medullary canal of the long bone and an opening 331
in communication with the chamber 330. The chamber 330 can be packed with
an osteogenic material to promote fusion while the cortical body 305 of
the dowel 300 provides support. The dowels are also advantageous in that
they provide desirable biomechanics and can be machined for various
surface features such as threads or annular ribbing. In some embodiments,
the outer cortical surface 310 of the tool engagement end 301 is machined
with an instrument attachment feature and an alignment score mark. As
shown in FIG. 3, the insertion end 302 may include a chamfered portion
 While these diaphysial cortical dowels are a major advance in this
field, a need has remained for bone dowels and other intervertebral
spacers with greater versatility.
SUMMARY OF THE INVENTION
 This invention provides spacers having an open chamber, tools
implanting the spacers and methods for making and using the spacers. The
spacers include a body having a wall which defines a chamber and an
opening in communication with the chamber. In one aspect, a channel is
defined in the wall in communication with the chamber and the outer
surface of the spacer. In another embodiment the wall includes a pair of
arms facing one another and forming a mouth to the chamber. In a
preferred embodiment, one of the arms is truncated relative to the other.
In some aspects, the body is composed of bone. In one aspect the body is
a dowel having a substantially C-shaped chamber and comprising an
off-center bone plug obtained from the diaphysis of a long bone.
 Tools for implanting spacers are also provided. The tools
spacer engaging means for engaging a spacer and occlusion means for
blocking an opening defined in the spacer. In one aspect the engaging
means includes a shaft slidingly disposed within a housing and having a
threaded post for engaging a threaded tool hole in the spacer. In some
embodiments, the occlusion means includes a plate extendable from the
housing. In one specific embodiment the plate defines a groove which is
disposed around a fastener attached to the housing so that the plate is
slideable relative to the housing.
 This invention also includes methods for obtaining an open bone
dowel and methods for using the spacers of this invention. The methods of
making a dowel according to this invention include cutting an off-center
plug from the diaphysis of a long bone to obtain a bone dowel having an
open chamber. In one aspect, the dowel is machined to include desirable
surface features such as threads, grooves and instrument holes. In still
another aspect, the methods include chamfering the forward end of the
dowel. The methods for using the spacers of this invention include making
a cavity between two vertebrae to be fused and implanting a spacer having
an open chamber. In some embodiments the chamber is packed with
osteogenic material before the spacer is implanted. In other aspects of
the invention, osteogenic material is packed into and around the chamber
through the mouth or channel after implantation.
 The combination of the open-chambered spacers of this invention
with the tools and methods of this invention provide a versatile spacer
without any compromise in biomechanical integrity. The spacers can be
packed before or after implantation. This invention facilitates
implanting a pair of open spacers close to each other in an
intervertebral space. Where the spacer is a bone dowel, the dowel can be
formed with less bone than is needed for conventional dowels, conserving
precious bone stock.
 Accordingly, it is one object of this invention to provide an
open-chambered fusion spacer and methods for using the spacer in an
 Another object is to improve patient incidence of safe and
satisfactory spinal stabilization and fusion.
 Another object of this invention is to provide a dowel for
vertebral fusions which has improved biomechanical properties and
versatility over standard dowels known in the art.
 Still another object of the present invention is to provide a
spacer with satisfactory biomechanical features and improved osteogenic
and fusion promoting features.
 These and other objects, advantages and features are accomplished
according to the spacers,
tools and methods of the following description
of the preferred embodiments of the present invention.
BRIEF DESCRIPTION OF THE DRAWINGS
 FIG. 1 shows a standard Cloward Dowel known in the art.
 FIG. 2 shows a standard unicortical dowel known in the art.
 FIG. 3 shows a diaphysial cortical dowel produced and sold by The
University of Florida Tissue Bank, Inc.
 FIG. 4 is a side perspective view of one embodiment of the
open-chambered spacer of this invention.
 FIG. 5 is an end elevational view of the spacer of FIG. 4.
 FIG. 6 is a top elevational view of a pair of open chambered dowels
of this invention implanted within an intervertebral space.
 FIG. 7 depicts the anatomy of a lumbar vertebral segment.
 FIG. 8 is a top elevational view of a pair of open chambered dowels
of this invention implanted within an intervertebral space via an
anterior surgical approach.
 FIG. 9 is a top elevational view of a pair of open chambered dowels
of this invention implanted within an intervertebral space via a
posterior surgical approach.
 FIG. 10 is a side perspective view of one embodiment of an open
chambered dowel having a truncated arm defining a channel to the mouth
 FIG. 11 is a top perspective view of an open chambered dowel with
arms defining concave faces.
 FIG. 12 is a top perspective view of an open chambered bone dowel.
 FIG. 13 is a side perspective view of one embodiment of this
invention in which the dowel is grooved.
 FIG. 14 is a side perspective view of a threaded dowel of this
 FIG. 15 is a side cross-sectional view of a detail of a portion of
the threads of a spacer of this invention.
 FIG. 16 shows various cuts across bone diaphysis and the resulting
dowels formed according to this invention.
 FIG. 17 is a top elevational view of one embodiment of a dowel
threader of this invention.
 FIG. 18 is a side elevational view of the dowel threader of FIG.
 FIG. 19 is an end elevational view of the dowel threader of FIGS.
17 and 18 showing elements of the cutter assembly.
 FIG. 20 is a detailed view of a single tooth of one cutter blade of
the dowel threader.
 FIG. 21 is a global side view of a cutter blade.
 FIG. 22 is a detailed side view of the cutter blade of FIG. 21.
 FIG. 23 is a detailed side view of the cutter blade of FIGS. 21 and
 FIG. 24 is a top perspective view of one embodiment of an insertion
tool of this invention.
 FIG. 25 is a side perspective view of the tool of FIG. 24.
 FIG. 26 is a perspective view of a spacer engaging element of an
 FIG. 27 is a perspective view of a spacer engaging element of an
 FIG. 28 is a side elevational view of an insertion tool engaged to
 FIG. 29 is a top perspective view of the view shown in FIG. 28.
 FIG. 30 is an exploded side perspective view of a tool-spacer
assembly according to this invention.
 FIG. 31 is a side perspective view of a tool-spacer assembly.
 FIG. 32 is a top perspective view of a fastener of this invention.
 FIG. 33 is a side elevational view of the fastener of FIG. 32.
 FIG. 34 is a top elevational view of the fastener of FIGS. 32 and
 FIG. 35 is a top elevational view of a spacer according to one
specific embodiment of this invention.
 FIG. 36 is a side view of the spacer of FIG. 35.
 FIG. 37 is a front perspective view of the spacer of FIG. 35.
 FIG. 38 is a detail of a portion of the threaded surface of the
spacer of FIG. 35.
 FIG. 39 is a detail of one embodiment of the thread of one
embodiment of the threaded dowel of this invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
 For the purposes of promoting an understanding of the principles of
the invention, reference will now be made to the embodiments illustrated
in the drawings and specific language will be used to describe the same.
It will nevertheless be understood that no limitation of the scope of the
invention is thereby intended, such alterations and further modifications
in the illustrated device, and such further applications of the
principles of the invention as illustrated therein being contemplated as
would normally occur to one skilled in the art to which the invention
 This invention provides spacers having an open-mouthed chamber.
These spacers are advantageous for maximum exposure of vertebral tissue
to osteogenic material within the chamber and allow close placement of a
pair of spacers within the intervertebral space. The design of these
spacers conserve material without compromising biomechanical properties
of the spacer. This is particularly advantageous when the material is
bone because the invention preserves precious allograft. In fact, larger
dowels and other shaped grafts can be obtained from smaller bones than
was ever thought possible before the present invention. Likewise, smaller
dowels having a pre-formed chamber may be efficiently obtained from
 Although any open-chambered spacer is contemplated, in one
embodiment the spacers are obtained as an off-center transverse plug from
the diaphysis of a long bone. This results in a dowel having an
open-mouthed chamber. Because the long bone naturally includes the
medullary canal, a pre-formed chamber is inherently contained within the
dowel. When the plug is cut off-center in a certain way, the dowel
includes an open-mouthed chamber. Surprisingly, the biomechanical
properties of these dowels are not compromised by the absence of the
missing chamber wall.
 Referring now to FIGS. 4 and 5, one embodiment of an interbody
fusion spacer of this invention is shown. The spacer 500 includes a body
505 with a tool engagement end 501 and an opposite insertion end 502. The
body 505 includes a wall 506 defining a chamber 530 between the two ends
501, 502 and an opening 531 in communication with the chamber 530.
Preferably, the insertion end 502 includes a solid protective wall 503
which is positionable to protect the spinal cord from escape or leakage
of osteogenic material from the chamber 530 when the spacer is placed via
an anterior approach.
 As shown in FIG. 4, the chamber 530 is open in that it also
communicates with a further aperture such as a mouth or a channel. The
aperture also communicates with the outer surface 510 of the spacer 500,
preferably at the tool engagement end 501. The aperture can provide
access to the chamber 530 after implantation or can facilitate insertion
of the spacer 530 into the intervertebral space. Comparing FIG. 4 with
FIG. 3, it is evident that the chamber 530 is open so that the body 505
and chamber 530 are substantially C-shaped as opposed to the defined
chamber 330 of FIG. 3. In some embodiments, including the one depicted in
FIG. 4, the aperture is a mouth 525 formed by a pair of facing and
opposing arms 520, 521.
 Bilateral placement of dowels 500 is preferred as shown in FIG. 6.
This configuration provides a substantial quantity of bone graft
available for the fusion. The dual bilateral cortical dowels 500 result
in a significant area of cortical bone for load bearing and long-term
incorporation via creeping substitution, while giving substantial area
for placement of osteogenic autogenous bone which will facilitate boney
bridging across the disc space. The dual dowel placement with facing
chambers 530 results in an elongated compartment 540 that can be filled
with an osteogenic composition M. This provides for the placement of a
significant amount of osteogenic material as well as a large support area
of cortical bone for load bearing.
 The open spacers of this invention are advantageous because they
complement the anatomy of the vertebrae V as shown in FIGS. 7-9. FIG. 7
shows the variation in bone strength within the vertebral body V, with
weaker bone W, disposed toward the center of the body B, and stronger
bone S being disposed around the periphery, closest to the ring apophysis
A. The open spacers of this invention are designed to accommodate spinal
anatomy. As shown in FIG. 8, two open spacers 500' can be implanted with
the mouths 525' facing to the center of the intervertebral space. This
capitalizes on the load bearing capability of the stronger peripheral
bone S of the vertebral body V by placing the structural and load bearing
portion of the spacer along the periphery of the body. At the same time,
the osteogenic material M placed within the chambers is exposed to the
more vascular center area W of the body.
 In a preferred embodiment shown in FIG. 10, the first arm 520' or
the arm adjacent the tool engagement end 501', is truncated relative to
the second arm 521'. This forms a channel 526 from the outer surface 510'
to the chamber 530'. Preferably and as shown in FIG. 10, the channel 526'
is in communication with both the mouth 525' and the chamber 530'
although it is contemplated that the channel 526 could be provided in a
closed spacer having a chamber and an opening. In some embodiments, such
as the spacer 550 depicted in FIG. 11, the arms 580, 581 define concave
faces or surfaces 582 and 583. The concave faces 582 and 583 are
configured to receive a complementary driving tool.
 The channel 526 of this invention provides important advantages.
The channel 526, particularly when formed as a truncated arm 520' as
shown in FIG. 10, facilitates implantation with an insertion tool.
Because the tool can be placed within the channel during implantation,
two spacers of this invention can be placed very closely together within
the intervertebral space as shown in FIGS. 6 and 8. The tool need not
extend beyond the outer surface of the spacer. The channel 526 also
allows osteogenic material to be packed within the chamber and around the
spacer after implantation. A further advantage of the channel is that,
when it is formed in combination with the mouth of an open spacer, it
allows the chamber of the spacer to be packed before implantation. The
tool may be placed within the channel to prevent escape of the osteogenic
material from the chamber during implantation. The channel 526 also
provides access to the chamber 530' for packing after the spacer 500' is
implanted into the disc space.
 Referring now to FIG. 12, in a preferred embodiment, the spacer is
a dowel having a longitudinal axis A.sub.1 along a length L of the body
505. The open C-shaped chamber 530 is defined along a second axis A.sub.p
substantially perpendicular to the longitudinal axis A.sub.1. The body
505 has an outer cross-section XS projected on a plane perpendicular to
the longitudinal axis A.sub.1 that is substantially uniform along the
length L of the body 505.
 The spacers of this invention may be provided with surface features
defined in the outer surface 510. Where the spacer is a bone dowel as
described herein, the surface features can be machined into the cortical
bone. Any desirable surface feature is contemplated. In one embodiment
the outer surface 510 of the tool engaging end 501 defines a tool
engaging or instrument attachment hole 515 as shown in FIGS. 4 and 12. In
a preferred embodiment, the hole 515 is threaded but any suitable
configuration is contemplated. It is sometimes preferable that this end
501 have a generally flat surface to accept the instrument for insertion
of the dowel in the recipient.
 In some embodiments, the spacer 500 includes an alignment score
mark or groove 516 defined in the tool engagement end 501. In FIG. 12 the
groove 516 is parallel to the axis A.sub.p of the chamber 530 or
perpendicular as shown in FIG. 4. The score mark may be widened to form a
driver slot for receiving an implantation tool. Alternatively, the end of
the dowel may be machined to exhibit a projection instead of a slot. Such
a protruding portion of bone may take a straight, flat-sided shape
(essentially a mirror image of the slot shown), it may be an elliptical
eminence, a bi-concave eminence, a square eminence, or any other
protruding shape which provides sufficient end-cap or tool engaging end
strength and drive purchase to allow transmission of insertional torque
without breaking the dowel or the eminence. In other embodiments, a
groove can be omitted to enhance the strength of the tool engaging end
 Other surface features can be defined along the length L of the
spacer. The surface features can provide engaging surfaces to facilitate
engagement with the vertebrae and prevent slippage of the spacer as is
sometimes seen with a smooth graft. Referring now to FIG. 13, the spacer
600 includes a groove or stop rib 632 inscribed along the circumference
of the spacer. The rib 632 discourages backing out. In other preferred
embodiments the outer surface 510' of the dowel 500" defines threads 542
as shown in FIG. 14. The initial or starter thread 547 is adjacent the
protective wall 503'. As shown more clearly in FIG. 15, the threads 542
are preferably uniformly machined threads which include teeth 543 having
a crest 544 between a leading flank 545 and an opposite trailing flank
546. Preferably the crest 544 of each tooth 543 is flat. In one specific
embodiment, the crest 544 of each tooth 543 has a width w of between
about 0.020 inches and about 0.030 inches. The threads 542 preferably
define an angle .alpha. between the leading flank 545 and the trailing
flank 546 of adjacent ones of said teeth 543. The angle .alpha. is
preferably between about 50 degrees and 70 degrees. Each tooth 543
preferably has a height h' which is about 0.030 inches and about 0.045
 Machined surfaces, such as threads, provide several advantages that
were previously only available with metal implants. Threads allow better
control of spacer insertion than can be obtained with a smooth surface.
This allows the surgeon to more accurately position the spacer and avoid
over-insertion which is extremely important around the critical
neurological and vascular structures of the spinal column. Threads and
the like also provide increased surface area which facilitates the
process of bone healing and creeping substitution for replacement of the
donor bone material and fusion. These features also increase
postoperative stability of the spacer by engaging the adjacent vertebral
endplates and anchoring the spacer to prevent expulsion. Surface features
also stabilize the bone-spacer interface and reduce micromotion to
facilitate incorporation and fusion.
 Various configurations of open-chambered spacers are contemplated
by this invention. When the spacer is obtained from the diaphysis of a
long bone, the shape of the dowel is determined by the location of the
cut into the bone shaft. Referring now to FIG. 16, by appropriately
locating the plug that is cut, "C"-shaped dowels of varying "C"-shaped
cavity depths and sidewall thicknesses are achievable. FIG. 16A shows the
plug that must be cut into the shaft to obtain a diaphysial cortical
dowel 300 (see FIG. 3) having a sidewall height H1 and a sidewall
thickness T1. FIGS. 16B-16D depict the off-center cuts required to
generate "C"-shaped dowels of this invention having different sidewall
heights H2-H4 and sidewall thicknesses T2-T4. The sidewall thickness
increases from 16A to 16D, even though the diameter of the dowel is
 Surprisingly, we have found that the open chambered spacers of this
invention have biomechanical properties similar to a spacer having a
defined or closed chamber. For example, the open-chambered bone dowel
500" of FIG. 14 is no more susceptible to snapping or breakage during
machining or implantation than the diaphysial cortical dowel 300 of FIG.
3 having a full circular chamber. This strength is retained as long as
the thickness T4 of the wall 506' at its narrowest aspect 570 is
preferably no less than about 5 mm.
 As any of these open-chambered spacers are implanted and begin to
experience axial load, it is expected that the lower the sidewall height
H, the greater the load carried by the dowel end 501, 502. However, where
the sidewall height H is approximately the same as the dowel diameter D,
the sidewall 506 may carry a greater share of this axial load.
 In some embodiments, the wall 506 may include upper and lower
flattened portions to stabilize the dowel by neutralizing any rotational
torque that may be induced by pressure on the sidewall. This could be
achieved by reducing the height H of the sidewall 505 and ends 501, 502
by filing or like means. These considerations may be less important for a
threaded dowel than a non-threaded dowel as the threads tend to "bite"
into the bone in which they are implanted, thereby preventing any
 For cervical fusions, the dowel is preferably obtained from the
fibula, radius, ulna or humerus. The dimensions of such dowels are
typically between about 8-15 mm in length or depth and about 10-14 mm in
diameter. For thoracic and lumbar fusions, the dowel is preferably
obtained from the humerus, femur or tibia. The dimensions of such dowels
are typically between about 10-30 mm in length and about 14-20 mm in
 The chamber may be packed with any suitable osteogenic material. In
a preferred embodiment, the osteogenic composition M has a length which
is greater than the length of the chamber 530 so that the osteogenic
composition will contact the endplates of the adjacent vertebrae when the
spacer 500 is implanted within the vertebrae. This provides better
contact of the composition with the endplates to stimulate bone ingrowth.
 Any suitable osteogenic material or composition is contemplated,
including autograft, allograft, xenograft, demineralized bone, synthetic
and natural bone graft substitutes, such as bioceramics and polymers, and
osteoinductive factors. The terms osteogenic material or osteogenic
composition used here means virtually any material that promotes bone
growth or healing including autograft, allograft, xenograft, bone graft
substitutes and natural, synthetic and recombinant proteins, hormones and
 Autograft can be harvested from locations such as the iliac crest
using drills, gouges, curettes and trephines and other tools and methods
which are well known to surgeons in this field. Preferably, autograft is
harvested from the iliac crest with a minimally invasive donor surgery.
The osteogenic material may also include bone reamed away by the surgeon
while preparing the end plates for the spacer.
 Advantageously, where autograft is chosen as the osteogenic
material, only a very small amount of bone material is needed to pack the
chamber. The autograft itself is not required to provide structural
support as this is provided by the spacer. The donor surgery for such a
small amount of bone is less invasive and better tolerated by the
patient. There is usually little need for muscle dissection in obtaining
such small amounts of bone. The present invention therefore eliminates or
minimizes many of the disadvantages of employing autograft.
 Natural and synthetic graft substitutes which replace the structure
or function of bone are also contemplated for the osteogenic composition.
Any such graft substitute is contemplated, including for example,
demineralized bone matrix, mineral compositions and bioceramics. As is
evident from a review of An Introduction to Bioceramics, edited by Larry
L. Hench and June Wilson (World Scientific Publishing Co. Ptd. Ltd, 1993,
volume 1), there is a vast array of bioceramic materials, including
BIOGLASS.RTM., hydroxyapatite and calcium phosphate compositions known in
the art which can be used to advantage for this purpose. That disclosure
is herein incorporated by reference for this purpose. Preferred calcium
compositions include bioactive glasses, tricalcium phosphates and
hydroxyapatites. In one embodiment, the graft substitute is a biphasic
calcium phosphate ceramic including tricalcium phosphate and
 In some embodiments, the osteogenic compositions used in this
invention comprise a therapeutically effective amount to stimulate or
induce bone growth of a substantially pure bone inductive or growth
factor or protein in a pharmaceutically acceptable carrier. The preferred
osteoinductive factors are the recombinant human bone morphogenetic
proteins (rhBMPs) because they are available in unlimited supply and do
not transmit infectious diseases. Most preferably, the bone morphogenetic
protein is a rhBMP-2, rhBMP-4 or heterodimers thereof.
 Recombinant BMP-2 can be used at a concentration of about 0.4 mg/ml
to about 1.5 mg/ml, preferably near 1.5 mg/ml. However, any bone
morphogenetic protein is contemplated including bone morphogenetic
proteins designated as BMP-1 through BMP-13. BMPs are available from
Genetics Institute, Inc., Cambridge, Mass. and may also be prepared by
one skilled in the art as described in U.S. Pat. Nos. 5,187,076 to Wozney
et al.; 5,366,875 to Wozney et al.; 4,877,864 to Wang et al.; 5,108,922
to Wang et al.; 5,116,738 to Wang et al.; 5,013,649 to Wang et al.;
5,106,748 to Wozney et al.; and PCT Patent Nos. WO93/00432 to Wozney et
al.; WO94/26893 to Celeste et al.; and WO94/26892 to Celeste et al. All
osteoinductive factors are contemplated whether obtained as above or
isolated from bone. Methods for isolating bone morphogenetic protein from
bone are described in U.S. Pat. No. 4,294,753 to Urist and Urist et al.,
81 PNAS 371, 1984.
 The choice of carrier material for the osteogenic composition is
based on biocompatibility, biodegradability, mechanical properties and
interface properties as well as the structure of the load bearing member.
The particular application of the compositions of the invention will
define the appropriate formulation. Potential carriers include calcium
sulphates, polylactic acids, polyanhydrides, collagen, calcium
phosphates, polymeric acrylic esters and demineralized bone. The carrier
may be any suitable carrier capable of delivering the proteins. Most
preferably, the carrier is capable of being eventually resorbed into the
body. One preferred carrier is an absorbable collagen sponge marketed by
Integra LifeSciences Corporation under the trade name Helistat.RTM.
Absorbable Collagen Hemostatic Agent. Another preferred carrier is a
biphasic calcium phosphate ceramic. Ceramic blocks are commercially
available from Sofamor Danek Group, B. P. 4-62180 Rang-du-Fliers, France
and Bioland, 132 Rou d Espangne, 31100 Toulouse, France. The
osteoinductive factor is introduced into the carrier in any suitable
manner. For example, the carrier may be soaked in a solution containing
 The present invention also provides methods for making the open
spacers of this invention. In one embodiment, a method for making an open
chambered bone dowel includes obtaining an off-center plug from the
diaphysis of a long bone so that the dowel has an open chamber. The open
chamber is preferably substantially concave or C-shaped and has an axis
that is substantially perpendicular to the long axis of the dowel.
Appropriate human source bones include the femur, tibia, fibula, humerus,
radius and ulna. Long bones from other species are also contemplated
although human source bones are generally preferred for human recipients.
 The first step is to identify an acceptable donor based upon
appropriate standards for the particular donor and recipient. For
example, where the donor is human, some form of consent such as a donor
card or written consent from the next of kin is required. Where the
recipient is human, the donor must be screened for a wide variety of
communicable diseases and pathogens, including human immunodeficiency
virus, cytomegalovirus, hepatitis B, hepatitis C and several other
pathogens. These tests may be conducted by any of a number of means
conventional in the art, including but not limited to ELISA assays, PCR
assays, or hemagglutination. Such testing follows the requirements of:
(i) American Association of Tissue Banks, Technical Manual for Tissue
Banking, Technical Manual--Musculoskeletal Tissues, pages M19-M20; (ii)
The Food and Drug Administration, Interim Rule, Federal Register/Vol. 58,
No. 238/Tuesday, Dec. 14, 1994/Rules and Regulations/65517, D. Infectious
Disease Testing and Donor Screening; (iii) MMWR/Vol. 43/No. RR-8m
Guidelines for Preventing Transmission of Human Immunodeficiency Virus
Through Transplantation of Human Tissue and organs, pages 4-7; (iv)
Florida Administrative Weekly, Vol. 10, No. 34, Aug. 21, 1992,
59A-1.001-01459A-1.005(12)(c), F.A.C., (12)(a)-(h), 59A-1.005 (15),
F.A.C., (4)(a)-(8). In addition to a battery of standard biomechanical
assays, the donor, or their next of kin, is interviewed to ascertain
whether the donor engaged in any of a number of high risk behaviors such
as having multiple sexual partners, suffering from hemophilia, engaging
in intravenous drug use, etc. Once a donor has been ascertained to be
acceptable, the bones useful for obtention of the dowels are recovered
 Preferably, the bone plugs are obtained using a diamond or hard
metal tipped cutting bit which is water cleaned and cooled. Commercially
available bits (e.g. core drills) having a generally circular nature and
an internal vacant diameter between about 10 mm to about 20 mm are
amenable to use for obtention of these bone plugs. Such core drills are
available, for example, from Starlite, Inc. In one embodiment, a
pneumatic driven miniature lathe having a spring loaded carriage which
travels parallel to the cutter is used. The lathe has a drive system
which is a pneumatic motor with a valve controller which allows a desired
RPM to be set. The carriage rides on two runners which are 1.0 inch
stainless rods and has travel distance of approximately 8.0 inches. One
runner has set pin holes on the running rod which will stop the carriage
from moving when the set pin is placed into the desired hole. The
carriage is moveable from side to side with a knob which has graduations
for positioning the graft. A vice on the carriage clamps the graft and
holds it in place while the dowel is being cut. The vice has a cut-out
area in the jaws to allow clearance for the cutter.
 In operation, the carriage is manually pulled back and locked in
place with a set pin. The graft is loaded into the vice and is aligned
with the cutter. Sterile water is used to cool and remove debris from
graft and/or dowel as the dowel is being cut. The water travels down
through the center of the cutter to irrigate as well as clean the dowel
under pressure. After the dowel is cut, sterile water is used to eject
the dowel out of the cutter.
 Dowels of any size can be prepared according to this invention. In
some embodiments, the dowels range from 5 mm to 30 mm diameters with
lengths of about 8 mm to about 36 mm being generally acceptable, although
other appropriate gradations in length and diameter are available. For
cervical dowels, such as anterior cervical fusion or ACF dowels, lengths
of 8 mm, 9 mm, up to about 15 mm are generally desirable. Dowels of
differing diameter are most conveniently obtained as follows:
10.6-11 mm fibula
12 mm radius
14 mm ulna
14+ mm small humeri
 Dowels for thoracic and lumbar fusions, such as anterior thoracic
inner body fusion (ATIF) and anterior lumbar inner body fusion (ALIF)
dowels, respectively, having a depth of between about 10-36 mm, and
preferably between about 15-24 mm, are generally acceptable, depending on
the needs of a particular patient. Dowels of differing diameter for
thoracic and lumbar fusions are most conveniently obtained as follows:
14-16 mm humerus
16-18 mm femur
18-20 mm tibia
 While the foregoing diameters and source bones for such dowels is a
useful guide, one of the significant advances provided by this invention
is that the open-chambered dowel of this invention provides tremendous
flexibility with respect to the source bone used.
 Since the spacers of the preferred embodiment are obtained from
off-center transverse plugs across the diaphysis of long bones, each
dowel has the feature of having a substantially "C"-shaped chamber
through the dowel perpendicular to the length of the dowel formed by the
intersection of the natural intramedullary canal of the source bone and
the cutter blade as it forms the plug. The canal cavity in the long bone
is, in vivo, filled with bone marrow. In the standard Cloward Dowel and
unicortical dowels known in the art, no such natural cavity exists and
the cancellous bone that forms the body of such dowels tends to be too
brittle to accept machining of such a cavity. The dowels of this
invention, by the nature of their origin, inherently define such a
cavity. Naturally, based on this disclosure, those skilled in the art
will recognize that other bone sources could be used which do not have
the intramedullary canal, and if sufficient strength is inherent to the
bone, a cavity or chamber could be machined. In addition, it will be
appreciated from the instant disclosure that an existing diaphysial
cortical dowel (FIG. 3), available from the University of Florida Tissue
Bank, Inc., could be modified by machining one side of such a dowel until
one wall of the dowel is sufficiently abraded to "break-through", thereby
transforming the diaphysial cortical dowel into the "C"-shaped dowel of
this invention. Accordingly, such extensions of this invention should be
considered as variants of the invention disclosed herein and therefore
come within the scope of the claims appended hereto.
 The marrow is preferably removed from the intramedullary canal of
the diaphysial plugs and the cavity is cleaned, leaving the chamber. The
spacer may be provided to the surgeon with the chamber prepacked or empty
for the surgeon to pack during surgery. The cavity or chamber can then be
packed with an osteogenic material or composition.
 The plug is then machined, preferably in a class 10 clean room to
the dimensions desired. The machining is preferably conducted on a lathe
such as a jeweler's lathe, or machining tools may be specifically
designed and adapted for this purpose. Specific tolerances for the dowels
and reproducibility of the product dimensions are important features for
the successful use of such dowels in the clinical setting.
 In some embodiments, the forward end of the dowel which is to be
inserted into a cavity formed between adjacent vertebrae is chamfered.
The curvature of the chamfered end facilitates insertion of the dowel
into the intervertebral space. Chamfering can be accomplished by
appropriate means such as by machining, filing, sanding or other abrasive
means. The tolerance for the chamfering is fairly liberal and the desired
object is merely to round or slightly point the end of the dowel that is
to be inserted into the cavity formed between adjacent vertebrae to be
 In some embodiments, the invention includes methods for providing
surface features into the walls of the dowels. The methods may include
defining a tool or instrument attachment hole in an end of the dowel. The
hole may be drilled and preferably tapped. Preferably, the dowel will be
of such dimensions as to fit standard insertion
tools, such as those
produced by Sofamor Danek Group, Inc. (1800 Pyramid Place, Memphis, Tenn.
38132, (800) 933-2635). In addition, a score mark or driver slot may be
inscribed on the instrument attachment end of the dowel so that the
surgeon can align the dowel so that the chamber is parallel with the
length of the recipient's spinal column. The mark or slot allows the
surgeon to orient the dowel properly after the dowel is inserted and the
chamber is no longer visible. In the proper orientation, the endplates of
the adjacent vertebrae are exposed to osteogenic material in the chamber.
In some embodiments, the driver slot is omitted to preserve as much bone
stock, and therefore strength, in the end as possible.
 Surface features such as grooves and threads may be preferably
defined or inscribed on the outer cylindrical surface of the dowel.
Machining of such features on dowels known in the art is difficult if not
impossible due to the brittle cancellous nature of such dowels.
Accordingly, the dowels of this invention have the advantage of having
very good biomechanical properties amenable to such machining.
 Those skilled in the art will also recognize that any of a number
of different means may be employed to produce the threaded or grooved
embodiments of the dowel of this invention. However, one preferred
embodiment of a thread cutter 400 is depicted in FIGS. 17-23. The cutter
400 includes a drive shaft 402 for supporting a spacer and a cutter
assembly 420. The terminal end 406 of the drive shaft 402 includes a
spacer engager 407. In one embodiment and as best shown in FIG. 18, the
spacer engager 407 is a protruding element which matingly corresponds to
the driver slot on the tool end of the open-chambered spacers of this
invention. The drive shaft 402 can be turned to rotate and advance the
spacer incrementally through the cutter assembly 420 to inscribe a
feature such as a thread into the surface of the spacer.
 In one embodiment, the drive shaft 402 can be turned by a handle
401 rigidly attached to a first end 402a of the shaft 402. The drive
shaft 402 preferably is provided with a graduated segment means for
controlled incremental advancement of the drive shaft 402 upon rotation
of the handle 401. In this embodiment, the means is a threaded portion
403. Support means 404 and 405 are preferably provided for alignment and
support of the shaft 402. Each of the support means 404, 405 include a
wall 404a, 405a defining an aperture 404b, 405b. The support means 404,
405 may having controlling means within the apertures 404b, 405b for
controlling rotation and incremental advancement of the shaft. In some
embodiments, the controlling means include matching threads or bearings.
 The thread cutter assembly includes a housing 408 and blades 421,
422 and guide plates 424, 425 mounted within the housing 408. The cutter
blades 421, 422 are held in place in the housing 408 by fixation wedges
423a and 423b while guide plates 424 and 425, having no cutting teeth,
are held in place by fixation wedges 423c and 423d. Fixation wedges
423a-d are held in place by screws 426a-d. The foregoing arrangement is
preferred, as it allows for easy assembly and disassembly of the cutter
assembly, removal of the cutter blades, cleaning of the various
components, and if desired, sterilization by autoclaving, chemical,
irradiative, or like means. The cutter blades 421, 422 and guide plates
424, 425 may be rigidly fixed in place by increasing the tension created
by tightening screws 426a-d, which draws the fixation wedges 423a-d into
the housing 408, thereby clamping these elements in place. Naturally,
based on this disclosure, those skilled in the art will be able to
develop equivalents of the cutter assembly system described herein, such
as by use of wing-nuts, welding or like means to affix these various
elements in appropriate cutting relationship to each other.
 Fixation wings 421c and 421d are provided to allow proper seating
of the cutter blade upon insertion into the housing 408. At .theta. a
line is provided on cutter blades 421 and 422, which allows for
appropriate registration between cutter blades 421 and 422 during
manufacture thereof. Upon insertion into the housing 408, it is critical
that the blades and the teeth thereon are appropriately registered so
that as blade 421 cuts into the bone dowel as it is rotationally advanced
through the cutter assembly 420, blade 422 is appropriately situated so
that its matching teeth are in phase with the thread inscribed by the
teeth on blade 421. This is accomplished by a combination of the fixation
wings 421d and 421c properly seating in the housing 408 such that wall
421c abuts the housing 408 and the housing 408 walls abut the insides of
wings 421d and 421c.
 The cutting edges 421a, 422a of the blades 421, 422 are disposed in
relation to each other so that they are on axis. The cutting edges 421a,
422a and the guiding edges 424a, 425a of the guide plates define an
aperture 427 for a spacer or dowel. The diameter of the dowel that may be
threaded according to this device is defined by the diameter of the
 The supports 404 and 405 and the housing 408 for the cutter
assembly are all preferably mounted on a steady, solid, weighty base unit
409 via screws, welding, or like attachment means at 410a-f. The supports
and the cutter assembly are configured so that there is an appropriate
travel distance 411 from the fully backed out terminal end of the drive
shaft 406 to the end of the cutter assembly 420. This distance must be
sufficient to allow insertion of a dowel blank and advancement of the
blank through the cutter assembly 420 to allow a fully threaded dowel to
emerge from the cutter assembly.
 The cutter maintains true tooth form from top to bottom, so that
the cutter can be sharpened by surface grinding the face. This is
achieved by wire-cutting the teeth such that there is about a 5.degree.
incline 62c between the descending vertices at the front and rear of each
tooth, and about an 8.degree. incline 62d between the front and rear of
the top of each tooth. This aspect can best be seen in FIG. 20. Also, the
thickness of the cutter blade, 62c, preferably about 0.100" can be seen
in that figure. The angle 61 in FIG. 20 is preferably about 60.degree..
The width of the top of the tooth 62b is preferably about 0.025". The
pitch 60 is preferably about 0.100". FIG. 21 shows an overall view of the
cutter blades 421 or 422 which are assembled in the cutter assembly
housing 408. The entire length of the cutter blade 421b is about 1.650".
 Details of the blades 421, 422 are shown in FIGS. 22 and 23. In
this embodiment, the cutter blade 421 has twelve cutting teeth 431-442.
The cutting edge 422a has eleven teeth 451-461 spread over the length of
the blade 422. At 451, the first tooth at 0.004" in this example is
encountered by the blank and at each successive tooth, an increase of
about 0.004" is made until the final tooth height of about 0.039 is
reached at 460 and 461. As a dowel blank is fed into the cutter assembly,
it first encounters a truncated tooth at 431, and at every subsequent
tooth, the height of the tooth is reached, in this example, of 0.039" at
441 and 442. The truncated teeth 431-440 feed into the dowel being cut
along the 30.degree. line so that the teeth cut on only two sides. The
dotted line 443 shows the final pitch and form that the cutter will cut
in the bone dowel.
 It will be recognized by those skilled in the art that all of the
foregoing elements should preferably be manufactured from durable
materials such as 440 stainless steel, or like materials. In particular,
the cutting surfaces 421a and 422a of the blades 421 and 422 are made
from hard metal.
 In operation, based on the foregoing description, it will be
appreciated that the cutter blades 421 and 422 are placed into the
housing 408, clamped into place via the fixation wedges 423a, b and the
screws 426a, b after the blades have been properly seated and the two
blades are perfectly aligned. A blank dowel is then loaded into the
orifice 427 and the drive shaft with the protruding element 408 is
inserted into a drive slot a dowel. As the handle 401 is turned, the
drive shaft forces the dowel to rotate and advance incrementally through
the cutter assembly 420, thereby inscribing the thread defined by the
cutter blades 421 and 422 into the outer cylindrical surface or
circumference of the dowel.
 As noted above, those skilled in the art will recognize that
modifications to the specifics of the device described will allow for the
preparation of the varied threads or grooves in the circumference of the
dowel. For example, to form a groove in a dowel, the dowel could be
mounted in a lathe, such as those known in the art and commercially
available, for example from SHERLINE PRODUCTS, INC., SAN MARCOS, Calif.
92069, and a cutter blade applied as the dowel is rotated.
 The final machined product may be stored, frozen or freeze-dried
and vacuum sealed as known in the art for later use.
 The spacers of this invention may be conveniently implanted with
known instruments and tools. Any instrument which will firmly hold the
implant and permit the implant to be inserted is contemplated.
Preferably, the instrument will be adapted to compensate for the open
structure of the spacers of this invention.
 The present invention further contemplates insertion devices for
facilitating the implantation of spacers, implants or bone graft. The
tools include spacer engaging means for engaging a spacer or other item
and occlusion means for blocking an opening defined in the spacer. One
embodiment of an insertion tool of this invention is depicted in FIGS.
 In one embodiment, an insertion tool 800 is provided which includes
a housing 805 having a proximal end 806 and an opposite distal end 807
and defining a passageway 810 between the two ends. A shaft 815 which has
a first end 816 and an opposite second end 817 is disposed within the
passageway 810. The first end 816 of the shaft 815 is adjacent the distal
end 807 of the housing 805. The first end 816 defines a spacer engager
819. An occlusion member 820 is attached to the housing 805.
 The spacer engager 819 has any configuration which will engage a
spacer. In some embodiments the spacer engager 819 includes a post 818 as
shown in FIG. 26 for engaging a hole in the spacer. The post 818 may have
any configuration which will provide for mating engagement with a hole in
a spacer. For example, in preferred embodiments, the engager 819 is
threaded as shown in FIG. 26 to matingly engage a threaded tool hole.
Other embodiments include sharply pointed tip 819 as shown in FIG. 24 or
a hexagonal shaped tip 819" (FIG. 27). In each case, the engager is
shaped and sized to mate engagingly with the tool hole of the spacer. In
other embodiments, the spacer engaging means is a pair of prongs having
opposite facing spacer engaging members for grasping an outer surface of
 The spacer insertion tool 800 also includes an occlusion member 820
for blocking an opening defined in the spacer when the spacer engager 819
is engaged to the spacer. In a preferred embodiment, the occlusion member
820 is extendable from the distal end 807 of the housing 805 for blocking
an opening in the spacer. As shown in FIG. 28, the occlusion member 820
closes the mouth 525' and channel 526 defined in the spacer 500'.
 The occlusion member 820 is preferably slideably engaged to the
housing 805. Referring now to FIG. 29, in one embodiment, the occlusion
member 820 includes a plate 821 which defines a groove 822. A fastener
830 is engaged to a fastener bore 809 in the housing 805 and the groove
822 is disposed around the fastener 830. In this way, the plate 821 is
slideable relative to the housing 805.
 As shown in FIG. 30, the housing 805 is preferably provided with a
recess 808 which is defined to accept the occlusion member 820 without
increasing the effective diameter of the device 800. The occlusion member
is also adapted for the best fit with the spacer. For example, the
interior surface 824 of the occlusion member would be curved to
complement the scalloped faces 582 and 583 shown in FIG. 11 for crescent
engagement. Referring now to FIGS. 30 and 31, the plate 821 of the
occlusion member 820 preferably includes a curved superior surface 825
which approximates and completes the minor diameter of the dowel 500'
when the spacer engager 819 is engaged to the tool engaging hole 515' and
the occlusion member 820 is blocking the channel 526 of the spacer 500'.
Preferably, the plate 821 and the arm 520' of the spacer 500' will be
configured such that the plate 821 will not extend beyond the channel 526
when the tool 800 is engaged to the spacer 500'. In other words, the
curved superior surface 825 will not increase the effective root diameter
RD of the the threaded outer surface 510'. This facilitates rotation and
screw insertion of the spacer and occlusion member combination into an
 The tool 800 depicted in FIG. 24 also includes a handle portion
840. The handle portion includes means for slidingly moving the shaft 815
within the housing 805 and for rotating the post 818. In the embodiment
shown in FIGS. 24 and 25 the means includes a thumbwheel 841. In some
embodiments, the handle portion 840 has a Hudson end attachment 842.
 Referring now to FIGS. 32-34, the fastener 830 is preferably
provided with a housing engaging means shown in FIG. 32 as a post 834,
and a plate engaging means or head portion 835. The fastener 830
preferably includes an internal hex 837 for receiving a fastener driving
tool. The post portion 834 may be threaded for mating engagement with
threaded bore 809 in the housing 805. In preferred embodiments shown in
FIGS. 29 and 30, the plate 821 defines a recess 826 surrounding the
groove 822. The diameter d1 of the head portion 835 is greater than the
diameter d2 of the post 834. The diameter d2 is less than the width w1 of
the groove 822. The diameter d1 of the head portion is greater than width
w1 but preferably no greater than the distance w2 between the outer edges
827 of the recess 826. Thus, the head portion 835 of the fastener 830 can
rest on the recess 826 while the post portion 834 extends through the
groove 822. In this way, plate 821 is slidable relative to the housing
805. This also provides for a low profile device which can be inserted
into various cannula for percutaneous procedures.
 The spacers and tools in this invention can be conveniently
incorporated into known surgical, preferably minimally invasive,
procedures. The spacers of this invention can be inserted using
laparoscopic technology as described in Sofamor Danek USA's Laparoscopic
Bone Dowel Surgical Technique, .COPYRGT. 1995, 1800 Pyramid Place,
Memphis, Tenn. 38132, 1-800-933-2635, preferably in combination with the
insertion tool 800 of this invention. Spacers of this invention can be
conveniently incorporated into Sofamor Danek's laparoscopic bone dowel
system that facilitates anterior interbody fusions with an approach that
is much less surgically morbid than the standard open anterior
retroperitoneal approaches. This system includes templates, trephines,
dilators, reamers, ports and other devices required for laparoscopic
dowel insertion. Alternatively, a minimally invasive open anterior
approach using Sofamor Danek's open anterior bone dowel instrumentation
or a posterior surgical approach using Sofamor Danke's posterior approach
bone dowel instrumentation are contemplated.
 The present invention also includes methods for fusing adjacent
vertebrae. The spine may be approached from any direction indicated by
the circumstances. The vertebrae and the intervertebral space are
prepared according to conventional procedures to receive the spacer. A
spacer of the appropriate dimensions is selected by the surgeon, based on
the size of the cavity created and the needs of the particular patient
undergoing the fusion. The spacer is mounted on an instrument, preferably
via an instrument attachment hole. In one embodiment, an osteogenic
material is placed within the chamber of the spacer and the channel and
or mouth of the spacer is then blocked with an occlusion member of the
instrument. The spacer is then inserted into the cavity created between
the adjacent vertebra to be fused. The spacer is oriented within the
intervertebral space so the osteogenic material in the chamber is in
communication with the end plates of the vertebra. Once the spacer is
properly oriented within the intervertebral space, the occlusion member
of the instrument can be withdrawn from the spacer aperture and the
spacer engager is disengaged from the spacer.
 In some embodiments, osteogenic material is packed into the chamber
through the channel after implantation. In still other embodiments, a
second spacer is implanted into the intervertebral space. FIG. 8 depicts
placement of two dowels of this invention implanted from an anterior
approach, while FIG. 9 shows bilateral placement of dowels from a
posterior approach. In each case the channel 526 opens adjacent the tool
engaging end 501' allowing access to the chamber 530' from either the
anterior or posterior approach.
 The combination of spacers of this invention with the tools of this
invention allow the spacers to provide the benefits of an open spacer
without suffering any biomechanical disadvantage or increased fiddle
factor. The occlusion member 825 blocks the mouth or channel to lessen
the stress on the wall of the spacer for smooth insertion. The occlusion
member also allows the chamber to be packed with osteogenic material
before the spacers are implanted. Once the spacer is implanted and the
occlusion member is withdrawn, additional osteogenic material can be
packed into the chamber or around the spacers. In some procedures two
open spacers are packed with the mouths facing one another as depicted in
FIG. 8. The open mouth of the spacers along with the tools of this
invention allow the spacers to be packed closely together because
virtually no clearance is required for the insertion tool. The open mouth
also allows the chambers to be packed after the spacer is implanted. This
is greatly enhanced when one of the arms is truncated, leaving a channel
from outside the disc space to the chamber as shown in FIG. 10.
 It has been found that certain dimensions are preferred when a
spacer of this invention is a bone dowel. For the substantially
"C"-shaped chamber, 530, a regular or irregular hole having a diameter no
greater than about 0.551" (14 mm) is preferred with a minimum wall
thickness 570 at the root of the thread of preferably no less than about
5 mm. Those skilled in the art will recognize that the foregoing
specifics, while preferable, may be modified depending on the particular
surgical requirement of a given application.
 In another specific embodiment, depicted in FIGS. 35-38, the
diameter D1 of the dowel is 18 mm and the length L1 is 36 mm. In this
specific embodiment, the length L2 of the solid side is shorter than the
length of the open side L1 due to the natural curve of the bone. The
shorter length L2 is preferably at least 30% of the longer length L1. The
length of the truncated arm is preferably between about 50-85% of the
diameter of the dowel D1. In this embodiment, the insertion end of the
dowel includes a flattened portion F1. The length of the flattened
portion F1 is preferably at least 70% of the diameter of the dowel D1. As
best shown in FIG. 36, the depth E1a, E1b, E2a, E2b of the end-caps or
insertion end and tool engaging ends of the dowel are preferably at least
about 3 mm. The depth of the bevel B2 of the threads is preferably 1 mm
while the bevel angle B1 is preferably about 45 degrees. The depth of the
drive slot C2 is preferably about 1.5 mm deep and the width C1 is about
5.5 mm. The diameter D2 of the tapped instrument attachment hole is about
3.3 mm with T5 indicating the tapped thread.
 Various surface feature configurations are contemplated by this
invention. Referring now to FIG. 38, a detail of the thread of one
embodiment is provided. The thread pitch T1 is about 2.5 mm. The length
T2 of the top of each tooth of the thread is about 0.6 mm, the depth T4
of the thread is about 1 mm and the width T3 of the thread at the thread
root is about 0.8 mm. The outer thread angle A3 is about 60 degrees in
this embodiment. FIG. 39 shows a detail of a portion of a threaded dowel
of another embodiment which has ten right handed threads per inch at a
helix angle at the root diameter of about 2.8892.degree.. In this
specific embodiment, the pitch T1' is 0.100"; the thread angle A1' is
60.degree.; the thread crest width T2' is 0.025"; the thread height T4'
is 0.039"; and the radius of the various thread angle as it changes R is
typically about 0.010".
 While the foregoing description discloses specific aspects of this
invention, those skilled in the art will recognize that any of a number
of variations on the basic theme disclosed herein can be made. It is
contemplated that the spacers of this invention can be formed of any
suitable biocompatible material, including metals, ceramics, polymers,
composites, alloys and the like. Some embodiments include titanium,
stainless steel, and Hedrocel.RTM.. Thus, for example, differing shapes
can be made from the diaphysis of various bones and could be used for
other orthopaedic purposes than vertebral fusions. In addition, any of a
number of known bone treatments can be applied to the dowel of this
invention to alter its properties. For example, the methods disclosed in
U.S. Pat. Nos. 4,627,853; 5,053,049; 5,306,303 and 5,171,279 can be
adapted and applied to the invention disclosed herein. Accordingly the
disclosures of those patents is herein incorporated by reference for this
 Having described the dowel of this invention, its mode of
manufacture and use, the following specific examples are provided by way
of further exemplification and should not be interpreted as limiting on
the scope of the invention herein disclosed and claimed.
 Torsional Testing of "C"-shaped Dowel
 The C-shaped dowel of this invention was tested and the following
measurements made of the dowel's ability to withstand insertional torque.
The data presented here are for the 16 mm dowel. However, similar results
are expected for other lengths of the dowel of this invention. For each
dowel, a measured torque is applied to the dowel as it is maintained in a
stationary position. For biological insertion of dowels, torques no
higher than about 1 newton-meter are expected. The various dimensions
measured in the following table correspond to the dimension shown in
Diam. OD ID Height Calc. % diff.
(mm) (mm) Thick- Meas.- Failure Failure
Sample D1 W1 W1* H**
ness*** Calc. Torque Type
1 15.8 5.1 4.6 13.2 4.0
15 4.00 s. wall
2 15.8 5.5 4.8 13.2 4.0 19 3.5 s. wall
15.9 6.2 5.3 13.4 4.3 25 3.89 slot
4 15.9 7 6.3 14.1 5.1 23 4.95
5 15.6 5.8 5.4 13.6 4.5 21 5.2 s. wall
6 15.8 5.5 4.9
13.1 4.0 24 4.36 s. wall
7 15.7 5.8 5.4 13.4 4.3 27 4.00 slot
W1*= W1 - T4;
H**= see H1-H4, FIG. 16;
thickness***= theoretical calculations based on sidewall height, H
 From these data, it is clear that dowels of this invention are able
to withstand considerably more than the 1 newton-meter of torque required
to insert the dowel in physiological situations. From theoretical
calculations based on the sidewall height, the difference between the
calculated sidewall thickness and the measured thickness was found to be,
on average, about 22%, leading to the conclusion that only approximately
22% the measured thickness is cancellous bone, and the substantial
majority of the bone is cortical bone.
 Compression Testing:
 The "C"-shaped dowel of this invention was compressively tested and
the load to failure was measured. It is anticipated that loads no higher
than about 10,000 newtons are likely to be experienced in-place in the
vertebral column. Compression testing of several different "C"-shaped
dowels of this invention indicated that dowels of this invention survive
axial compression loads significantly higher than the 15,000 newton
Avg Avg Avg mass Failure
# Thread D1 L1 E2a
E2b E2 E1a E1b E1 g Load (N)
1 no 15.9 25.5 5.3 4.7 5.0
3.9 4.4 4.2 4.193 4372
2 yes 15.9 23.7 5.1 5.1 5.1 4.2 5.0 4.6
3 no 15.9 23.7 4.8 5.3 5.1 3.8 2.6 3.3 4.035 33748
4 yes 15.9 22.5 7.1 7.0 7.1 7.0 5.4 6.2 5.075 20940
16.0 23.4 5.6 5.4 5.5 5.8 6.2 6.0 4.986 22420
6 yes 15.7 26.1 7.1
7.1 7.1 8.4 8.6 8.5 5.331 24500
7 yes 16.8 23.8 5.4 5.0 5.2 6.0
6.0 6.0 3.928 14389
8 yes 17.6 22.4 4.8 5.5 5.2 5.8 4.6 5.2 5.448
9 poor 16.9 22.2 6.7 5.2 8.0 6.4 4.7 5.1 5.228 19576
10 poor 17.9 28.3 7.8 7.2 7.5 7.6 7.2 7.4 6.201 20606
11 yes 17.9
21.2 4.9 6.8 5.9 4.5 4.4 4.5 5.654 21461
12 yes 17.8 23.6 6.6 6.3
6.5 6.0 5.4 5.7 5.706 23971
13 yes 19.9 25.6 6.3 6.6 6.5 6.4 6.4
6.4 7.915 24761
 The mean load to failure of these dowels is 18544 newtowns,
indicating that on average, more dowels can withstand 15000 newtons axial
pressure than not. These data also indicate the need for diligent quality
control to eliminate dowels that do not withstand minimal axial
compression loads from being implanted.
 Cervical Fusion Using "C"-shaped Dowel
 Preoperative Diagnosis. Ruptured cervical disc and spondylosis
 Operative Procedure. Anterior cervical discectomy and fusion C5-6.
 After satisfactory general endotracheal anesthesia in the supine
position, the patient is prepped and draped in the routine fusion.
Incision is made in the skin length of the neck and carried through the
platysma muscle. Dissection is carried down to expose the anterior
vertebral column and the appropriate space identified by x-ray.
Discectomy and foraminotomy are then performed and there is found a
central, extruded fragment of disc toward the right side. When adequate
decompression is achieved, a "C"-shaped dowel is cut from bone bank
fibular and counter-sunk between the vertebral bodies to afford
distraction. The wound is then irrigated with Bacitracin and closed in
layers with Dexon and sterile strips.
 Postoperative evaluation and subsequent patient monitoring reveals
successful operative outcome and good vertebral fusion.
 It should be understood that the example and embodiments described
herein are for illustrative purposes only and that various modifications
or changes in light thereof will be suggested to persons skilled in the
art and are to be included within the spirit and purview of this
application and the scope of the appended claims.
* * * * *